Astrophysics

Abstract: We report the discovery of a 16th magnitude star, HE0437-5439, with a
heliocentric radial velocity of +723+-3km/s. A quantitative spectral analysis
of high-resolution optical spectra obtained with the VLT and the UVES
spectrograph shows that HE0437-5439 is a main sequence B-type star with
Teff=20350K, log g=3.77, solar within a factor of a few helium abundance and
metal content, rotating at v sin i=54km/s. Using appropriate evolutionary
tracks we derive a mass of 8 Msun and a corresponding distance of 61 kpc. Its
galactic rest frame velocity is at least 563km/s, almost twice the local
Galactic escape velocity, indicating that the star is unbound to the Galaxy.
Numerical kinematical experiments are carried out to constrain its place of
birth. It has been suggested that such hyper-velocity stars can be formed by
the tidal disruption of a binary through interaction with the super-massive
black hole at the Galactic center (GC). HE0437-5439 needs about 100Myrs to
travel from the GC to its presentposition, much longer than its main sequence
lifetime of 25Myrs. This can only be reconciled if HE0437-5439 is a blue
straggler star. In this case, the predicted proper motion is so small that it
can only be measured by future space missions. Since the star is much closer to
the Large Magellanic Cloud (LMC, 18kpc) than to the GC, it can reach its
position from the center of the LMC. The proper motion predicted in this case
is about 2mas/y (relative to the LMC), large enough to be measurable with
conventional techniques from the ground. The LMC origin could also be tested by
a high-precision abundance analysis.